Load lifting sling with built-in load indicator



R. NORTON 3,463,534

LOAD LIT'TING suns WITH BUILT-IN LOAD INDICATOR Aug. 26, 1969 2 Sheets-Sheet 1 Filed Oct. 5, 1967 75m N'urZ/cm Kna k/w z- 26. 1969 R. NORTON 3,463,534

LOAD LIFTING SLING WITH BUILT-IN LOAD INDICATOR Filed on. 5, 19s? 2 Sheets-Sheet 2 ZS T I I. I I

jmdew w Ray Nari an United States Patent 3,463,534 LOAD LIFTING SLING WITH BUILT-IN LOAD INDICATOR Roy Norton, Milwaukee, Wis., assignor to The Wear-Flex Corporation, Milwaukee, Wis., a corporation of Wisconsin Filed Oct. 5, 1967, Ser. No. 674,060

Int. Cl. B66c 1/18, 1/12 US. Cl. 294--74 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to load lifting slings and refers more particularly to slings made of fabric and especially nylon webbing. Examples of such fabric slings will be found in the Barthule Patent No. 2,903,291 and the Norton Patent No. 3,290,083.

Structurally the present invention is very much like that of my copending application Ser. No. 674,057, filed concurrently herewith, and which contains claims that cover much of the structure disclosed herein.

Load lifting slings-whether they are formed of fabric, as in this case, or otherwise-have definite limits in their load carrying capacity beyond which they cannot be safely used. Though every sling is designed and constructed to have a specific rated capacity, this does not assure against use of a sling for loads in excess of that capacity and often well into its safety factor. Time and again an operator is confronted with the problem of lifting a load of unknown weight, but even where the weight of the load to be lifted is known, it is seldom possible to predetermine the stress to which a lifting sling will be subjected in lifting that load. This follows from the fact that the stress in the sling depends not only upon the weight of the load but also upon the angle between the sling and the load. The smaller this sling-to-load angle becomes, the greater will be the stress in the sling; and, of course, the greater the stress, the less will be the load that can be safely handled by that sling.

To illustrate, a sling capable of safely handling a tenthousand pound (10,000 lb.) load on a direct upward pull in which the sling-to-load angle is ninety degrees (90), can only handle eight-hundred and seventy pounds (870 lbs.) with a five degree sling-to-load angle.

It is apparent, therefore, that some way of indicating when the tension load on a lifting sling approaches and finally reaches a tension load that is the maximum that can be safely handled by the sling would be a valuable adjunct to the sling. This invention fills that need.

It is not the purpose of this invention to merely provide an indication that the sling has been overloaded-as is the case in the sling of the Feiser Patent No. 2,966,878. Such after-the-event information, which usually is obtained during the periodic examinations of the sling, merely reveals that the sling has been overloaded andwhen this is discovered-the sling must be discarded as unsafe. In contrast, the indicator of this invention enables the operator to avoid overloading the sling.

3,463,534 Patented Aug. 26, 1969 It is therefore the purpose and object of the present invention to provide a load lifting sling having means incorporated therein for indicating when the load on the sling is approaching and finally reaches the maximum it can safely handle, as well as the fact that the sling has been overloaded.

More important, it is an object of this invention to provide a load lifting sling having indicating means incorporated therein by which the operator can tell if the load on the sling exceeds its rated capacity, even before the load is lifted off the floor.

All load lifting slings stretch more or less under tension, but slings made of nylon webbing and the like have an appreciable stretch factor. Such a sling-and, for that matter, the type of nylon webbing commonly used for slings per se, whether it be part of a load lifting sling or not-will stretch approximately one-third of its original length before it breaks. Thus, a three-foot sling will stretch to four feet before it breaks, and this stretch or elongation is uniformly proportional to the tension load so that at its rated capacity a three-foot n ylon sling having a one-to-five safety factor will be stretched to three feet, two and four-fifths inches (3 2%").

It is this characteristic of fabric webbing, and particularly nylon webbing which makes possible the indicator means of this invention.

With these observations and objects in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings. This disclosure is intended merely to exemplify the invention. The invention is not limited to the particular structure disclosed, and changes can be made therein without departing from the invention.

The drawings illustrate several complete examples of the physical embodiment of the invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

FIGURE 1 is a perspective view of an endless fabric sling equipped with the load indicator of this invention, illustrating the same about to lift a load;

FIGURE 2 is a side view of that portion of the sling shown in FIGURE 1 at which the load indicator is located, showing the same in its free untensioned condition;

FIGURE 3 is a view similar to FIGURE 2, but illustrating the condition which obtains when the sling is under a load slightly less than that for which the indicator is designed, and which may be the rated capacity of the sling;

FIGURE 4 is a perspective view of another type of fabric sling known in the trade as an eye-and-eye sling, likewise equipped with the load indicator of this invention;

FIGURE 5 is a perspective view of one end portion of an eye-and-eye sling such as that of FIGURE 4, but provided with a slightly modified form of load indicator;

FIGURE 6 is a perspective view of an end portion of another type of sling, having the load indicator of this invention embodied therein in still another form;

FIGURE 7 is a perspective view of one end of an eye-and-eye sling, illustrating a further modification of this invention;

FIGURE 8 is a fragmentary detail view partly in elevation and partly in section, illustrating the manner in which the load indicator of this invention may be used to produce a signal at a remote point; and

FIGURE 9 is a perspective view illustrating still another modification of this invention.

Referring now particularly to the accompanying drawings, and especially FIGURES 1, 2 and 3 thereof, the numeral 5 designates generally a fabric load lifting sling of the endless variety. In this case the sling consists of a single band or length of webbing-preferably nylon webbinghaving its end portions 6 and 7 overlapped and secured together by stitching 8. There are, of course, many ways to use such an endless sling, the manner of using it illustrated in FIGURE 1 being but illustrative. As there shown, two slings depending from the hook 9 of an overhead crane or the like, engage under spaced portions of the load 10. Thus the sling-to-load angle LA is considerably less than the optimum 90, which places a far greater strain on the slings than would a direct upward lift.

Upon elevation of the hook 9, the slings will stretch before the load 10 is lifted off the floor. If the weight of the load is within the capacity of the slings, all is well; but if the load exceeds the rated safe capacity of the sling, it may or may not be within the safety factor of the sling. Heretofore, there was no way of knowing whether or not the slings would be overloaded in lifting the loadand if perchance the tension force applied to the slings by the load closely approached the tensile strength of the slings, a most dangerous condition would exist since any slight increase in the stress on the slings resulting from any one of a host of unpredictable causes would result in rupture of the slings carrying the load.

To guard against such a dangerous possibility, each sling is equipped with a load indicator 11 by which the operator can tell if the load on the sling exceeds the safe limit, even before the load is lifted off the floor.

In its preferred form, this indicator comprises a flexible strap 12 having its ends fixed to the sling at endwise spaced points so that the strap 12 overlies a portion 13 of the sling. With the sling in its free unstretched condition shown in FIGURES 1, and 2, the strap 12 is substantially longer than the underlying portion 13 of the sling so that the strap is slack and displaced or bowed outwardly from the side of the sling. However, when a tension load is applied to the sling, its natural elongation stretches the underlying portion 13 and, in so doing, progressively takes up the slack in the strap 12 and if the load is great enough, the strap 12 is drawn into surface-to-surface contact throughout its length with the underlying portion 13.

It follows, therefore, that by properly selecting the differential in length between the strap 12 and the underlying portion 13, the initial surface-to-surface engagement therebetween can be caused to occur at any load. Preferably that load should be the rated capacity of the sling, which in order to provide an adequate safety factor usually is one-fifth of the breaking load.

Since nylon webbing is available with a calibrated stretch factor that identifies the amount the webbing will elongate before it ruptures, and the elongation is uniformly proportional with increasing tension, it is evident that selection of the proper differential in the length of the strap 12 and its underlying sling portion 13 to have the indicator identify when the load on the sling is at its rated capacity, is but a matter of arithmetic.

In that embodiment of the invention shown in FIG- URES 1, 2 and 3, the indicator strap 12 is an extension of the end portion 6 of the band forming the sling, having its extremity 14 secured to the underlying portion 7 of the sling by stitching or the like. The location of the point of securement of the extremity 14 is such that when the sling is free and unstretched, the strap 12 has the correct amount of slack. FIGURE 2 depicts this condition, while FIGURE 3 shows the slack almost fully taken up by the elongation of the sling under load.

In the eye-and-eye sling shown in FIGURE 4, the load indicator 11 is located at one end of the sling, and as in the endless sling of FIGURES l, 2 and 3, the indicator strap 12 is again an extension of the adjacent end of the band forming the sling.

In FIGURE 5, the indicator element or strap 12 encircles the entire eye 16 at one end of the sling, and has both its ends 17 secured to the lap which forms the eye 16. As shown, the strap 12 is longer than the circuma ference of the eye 16 when the sling is free and untensioned. This condition is exaggerated in FIGURE 5 for clarity of illustration.

FIGURE 6 shows the load indicator mounted on a fabric band 18 which may be a part of a sling or merely a load securing member such as used to tie down freight in an airplane. Thus in this case the indicator strap 19 is simply a length of any flexible material having its ends 20 secured to the band 18 at points spaced apart lengthwise of the band a distance which is less than the length of the medial unsecured portion of the strap when the band is free and untensioned.

In FIGURE 7 the load indicator is in the form of a piece 21 of material which has less tensile strength than the webbing of which the sling is made, the sling shown being of the eye-and-eye type. The piece 21 which may be considered a breakable link, is sandwiched between the overlapped portions 22 and 23 of the webbing, which are secured together by stitching 22 and 23 to form the eye. The stitching also passes through the end portions of the piece or link 21 to secure the same in place and form slack in the medial parts of the overlapped portions 22 and 23 so that these portions are incapable of carrying tension loads on the sling until the link 21 ruptures. Thus by making the piece or link of material having a tensile strength only sufficient to carry the rated capacity of the sling, the rupturing of the link will afford an unmistakable indication of the fact that the load on the sling exceeds its rated capacity; and if this occurs after the load has been lifted off the floor, the load will not be dropped but merely transferred to the overlapped portions 22 and 23.

While this fail safe attribute of the sling of FIGURE 7 and the unmistakable nature of the indication afforded by rupture of the link 21 has advantages, this form of the invention is not as desirable as those previously described, since indication of the overload results in desstruction of the indicator.

If desired, the indicator of FIGURES 1 to 6, inclusive, may be equipped with electrical contacts 24 and 25, as shown in FIGURE 8, to enable the information obtained by response of the indicator to be manifested at a remote location. Simply by connecting the contacts into an eletrical circuit including a source of current and an electric signal device, as shown, achieves this purpose.

In the various embodiments of the invention described hereinbefore, the indicator consists essentially of overlying flexible elements of unequal free length, secured together at their ends so that the longer element is slack until the shorter element has been stretched far enough to take up the slack. This is undoubtedly the preferred way of implementing the invention. It is possible, though, to achieve the purposes of the invention in another way which substitutes lost motion for slack. Thus as shown in FIGURE 9, the indicator may be an elongated rod 29 or the likeflexible if desiredhaving one end fixed to the webbing, as at 26, and its other end portion slidably received in a hole through an abutment 27 also fixed to the webbing. A shoulder provided by a collar 28 fixed on the unsecured extremity of the rod completes the structure. Obviously, when the webbing has been stretched far enough, the abutment and the shoulder will come into contact, and this will indicate that a predetermined tension load has been applied to the webbing. By making the collar 28 adjustable on the rod, as by having it slideable thereon and securing it in place by a set screw 30, the irlrdicator can be set for any load within the capacity of the s mg.

From the foregoing description taken in connection with the accompanying drawings, it will be apparent that this invention enables users of load lifting slings to avoid dangerously overloading the slings and completely eliminates guesswork in determining whether a given load can be safely lifted with a particular sling, all without damage to the sling.

What is claimed as my invention is:

1. A load lifting sling having a breakable part which is subjected to tension during use of the sling, and which breaks when the tension load on the sling is greater than can be safely handled by the sling, said sling being characterized by:

a flexible strap overlying said breakable part and having its ends secured thereto, the length of said flexible strap between its secured ends being greater than the breakable part it overlies,

so that said strap carries none of the load on the sling until said part breaks between the points of securement of the strap thereto, and said flexible strap having suflicient tensile strength to carry the load thereafter.

2. A load lifting sling having a lapped connection between two portions of the sling:

(A) a strap confined between said lapped portions, both lapped portions being longer than said strap and each having a medial slack section displaced outwardly from the strap, and end sections that lie flat against the end portions of the strap; and

(B) means securing said end sections together and to the end portions of the strap,

so that tension forces applied to the lapped connection during use of the sling are borne solely by the strap until the load thereon exceeds the tensile strength of the strap and ruptures the same, whereupon the load is transferred to the stitched together lapped portions of the sling.

3. The load lifting sling of claim 2, wherein the strap has sufficient tensile strength to carry a load that can be safely handled by the sling but insuflflcient tensile strength to withstand rupture before it is stretched far enough to take up all of the slack in the medial sections of said lapped portions,

so that an unsafe load on the sling is signaled by rupture of the strap.

4. A load lifting sling comprising:

(A) a strap of fabric webbing which has substantially uniform elongation per increment of increasing tension applied thereto,

an end portion of the strap overlying a lengthwise intermediate portion thereof to form a loop, which like the rest of the sling undergoes elongation upon being subjected to tension; and

(B) means on the sling for indicating when a load on the sling exceeds its rated capacity, said indicating means comprising:

(1) overlying stretches of webbing which are unequal in length,

(a) one of said unequal length stretches being an integral part of the strap and the other being an entirely separate piece of Webbing,

(b) the end portions of said unequal length stretches being in overlying relationship with each other, and

(c) there being a zone at which more than two plys of Webbing overlap resulting from said shorter stretch having at least one of its end portions interposed between an end portion of the longer stretch and an intermediate part of the strap, and

(2) stitching passing through the overlying end portions of said unequal length stretches to secure the same together,

(a) part of said stitching passing through said zone, and

(b) said stitching also securing the end portion of the strap to the intermediate portion thereof,

both of said unequal length stretches being capable of carrying tension loads applied to the sling, but the longer of the two strteches remaining free of any load on the sling until the shorter stretch has been elongated a predetermined amount,

so that the differential in length of said stretches as a tension load is applied to the sling is an indication of the extent of the load.

5. The load lifting sling of claim 4,

(A) wherein the shorter one of said unequal length stretches is that part of the strap that forms the loop, and

(B) wherein the longer one of said unequal length stretches is the separate piece of webbing and encircles the loop.

6. The load lifting sling of claim 4,

(A) wherein the stitching by which the end portion of the strap is secured to the lengthwise intermediate portion of the strap is confined to two separate areas spaced apart lengthwise of said end portion of the strap,

so that for a part of its length the end portion is not secured,

(B) wherein the longer one of said unequal length stretches is said unsecured part of the end portion of the strap, and

(C) wherein the shorter one of said unequal length stretches is the separate piece of webbing and is confined between said end portion of the strap and said lengthwise intermediate portion thereof.

7. A load lifting sling comprising a length of fabric webbing having an end portion thereof formed into a loop by which the sling may be attached to a crane hook or the like, and which loop as well as the remainder of the sling undergoes predetermined elongation per pound of load on the sling, characterized by indicator means on the sling by which an operator can tell when a load on the sling exceeds its rated capacity, said indicator means comprising:

a flexible strap encircling the loop and having its ends secured to one another and to the contiguous portions of the looped end portion of the sling,

said flexible strap being longer than the loop it encircles so that upon application of tension on the sling by means of its looped end, the encircling strap remains free of surface-to-surface engagement with the loop until the loop has undergone a predetermined elongation, whereby initial surface-to-surface engagement between the strap and the loop provides the desired indication.

References Cited UNITED STATES PATENTS 10/1945 Black 73-443 12/1966 Norton 29474 ANDRES H. NIELSEN, Primary Examiner 

